The Federal Code of Regulations (46 cfr 10) requires anyone taking consideration (money) from a passenger to be on a vessel on federal waters, to have a merchant mariner credentials (Captains License).

So what are federal waters? Normally if it is connected, so as the water flows to the ocean, it is federal waters. In general, federal waters begin three to nine nautical miles offshore to the 200 mile limit of the Gulf of Mexico. From Texas and Florida, federal waters begin nine nautical miles out, and from Mississippi, Louisiana and Alabama, federal waters begin three nautical miles out.

However, nothing is that clear cut. The US Army Corp of Engineers, the US EPA, and USCG are involved in designating federal waters.

Listed below are links to PDF files which indicate what waters are federal waters. This is not an inclusive list. If in doubt call the USCG headquarters office for that area.